SMISSING

[pspp] / doc / statistics.texi

diff --git a/doc/statistics.texi b/doc/statistics.texi
index 78b76bfbc492e2a1d48622ceb04f9bd629fd6e77..5b8b841933f3974b2f12410df414864a15b87523 100644 (file)
--- a/doc/statistics.texi
+++ b/doc/statistics.texi
@@ -1506,7 +1506,7 @@ order:
 
 @table @asis
 @item Explicit categories.
-@anchor{CTABLE Explicit Category List}
+@anchor{CTABLES Explicit Category List}
 To explicitly specify categories to include, list the categories
 within square brackets in the desired sort order.  Use spaces or
 commas to separate values.  Categories not covered by the list are
@@ -1707,9 +1707,46 @@ is optional.
 
 With @code{SMISSING=VARIABLE}, which is the default, missing values
 are excluded on a variable-by-variable basis.  With
-@code{SMISSING=LISTWISE}, when scalar variables are stacked, a missing
-value for any of the scalar variables causes the case to be excluded
-for all of them.
+@code{SMISSING=LISTWISE}, when stacked scalar variables are nested
+together with a categorical variable, a missing value for any of the
+scalar variables causes the case to be excluded for all of them.
+
+As an example, consider the following dataset, in which @samp{x} is a
+categorical variable and @samp{y} and @samp{z} are scale:
+
+@psppoutput{ctables18}
+
+@noindent
+With the default missing-value treatment, @samp{x}'s mean is 20, based
+on the values 10, 20, and 30, and @samp{y}'s mean is 50, based on 40,
+50, and 60:
+
+@example
+CTABLES /TABLE (y + z) > x.
+@end example
+@psppoutput{ctables19}
+
+@noindent
+By adding @code{SMISSING=LISTWISE}, only cases where @samp{y} and
+@samp{z} are both non-missing are considered, so @samp{x}'s mean
+becomes 15, as the average of 10 and 20, and @samp{y}'s mean becomes
+55, the average of 50 and 60:
+
+@example
+CTABLES /SMISSING LISTWISE /TABLE (y + z) > x.
+@end example
+@psppoutput{ctables20}
+
+@noindent
+Even with @code{SMISSING=LISTWISE}, if @samp{y} and @samp{z} are
+separately nested with @samp{x}, instead of using a single @samp{>}
+operator, missing values revert to being considered on a
+variable-by-variable basis:
+
+@example
+CTABLES /SMISSING LISTWISE /TABLE (y > x) + (z > x).
+@end example
+@psppoutput{ctables21}
 
 @node CTABLES Computed Categories
 @subsection Computed Categories
@@ -1722,7 +1759,7 @@ for all of them.
 categories created using arithmetic on categories obtained from the
 data.  The @code{PCOMPUTE} subcommand defines computed categories,
 which can then be used in two places: on @code{CATEGORIES} within an
-explicit category list (@pxref{CTABLE Explicit Category List}), and on
+explicit category list (@pxref{CTABLES Explicit Category List}), and on
 the @code{PPROPERTIES} subcommand to define further properties for a
 given postcompute.
 
@@ -1749,7 +1786,7 @@ postcompute must have the same form.
 @itemx MISSING
 @itemx OTHERNM
 These forms evaluate to the summary statistics for categories matching
-the given syntax, as described in previous sections (@pxref{CTABLE
+the given syntax, as described in previous sections (@pxref{CTABLES
 Explicit Category List}).  If more than one category matches, their
 values are summed.